I remember my dad putting a blanket over the Land Rover engine. It caught fire... all the wiring had to be replaced. There's not much else to burn... Oils are better now than in the '50's - and modern cars have much smaller water volumes so warm up faster. Core plugs are often mentioned as letting pressure out. But they are just core plugs - they plug the holes left by the supports that hold the inner cores in place when making hollow castings. They might accidentally help if the block freezes - but I'm pretty sure they weren't designed for that reason.